Free versus bound salicylate concentration in adults and children with chronic inflammatory joint disease: serum albumin and other potential factors affecting the levels.

Salicylate (SA) binding to albumin in purified human serum albumin (HSA) solutions was found to be predictably dependent on the concentrations of albumin and total SA. The influence of the presence of other ligands on SA binding in HSA solutions was measured and, in the case of bound fatty acids, free fractions of SA were increased by up to 100%. In sharp contrast, the free fraction of SA present in sera from patients with arthritis who were on long-term aspirin (ASA) therapy was roughly twice that found in HSA solutions. A correlation between serum concentrations of free SA and albumin was evident in sera from one group of children with juvenile rheumatoid arthritis (r = -0.75) but was not seen in sera from the other patients. This would indicate that in vivo, the albumin level is not the sole, or, in some patients, even the dominant determinant of the free fraction of SA. Additional factors such as fatty acids and other hydrophobic ligands, both endogenous and exogenous, should be studied further, especially in adults.